The invention relates to a sensor and evaluation system for acquiring data at a measuring point, especially for double sensors for detecting final positions and limit values, consisting of at least two sensors that are connected via a control circuit to a remotely located switching amplifier with a current source or voltage source, whereby a connection wire leads from the current source or voltage source to the sensor and another connection wire leads from the sensor to an evaluation unit, and the sensor signal of each sensor normally consists of a change in the current flow as compared to the current flow in the quiescent state of the sensor and said sensor signal is transmitted via the control circuit to the switching amplifier in order to be evaluated there in an evaluation unit and to be output at an output stage in the form of signals (e.g. binary ZERO, ONE) as a function of the sensor and of the time, according to the generic part of claim 1.
Sensors are normally connected by two connection cables or by a two-core connecting line to a switching amplifier which supplies the sensors with direct current or direct voltage. The sensor signal consists in a change in the strength of the direct current flowing through the sensor to the evaluation unit as compared to the current flow in the quiescent state of the sensor. For example, an oscillator that is integrated into the sensor vibrates when the current circuit is closed and, in the quiescent state, i.e. without an object to be measured, it consumes a certain current, which is measured as the quiescent signal. If an object to be measured is brought closer to the active surface of the sensor, then the current consumption of the sensor changes, for example, as a result of eddy-current losses in the object to be measured, and thus the current flow in the evaluation unit also changes. The sensor signal is measured and amplified in the evaluation unit, and an output stagexe2x80x94depending on the signal level, e.g. when the value exceeds or falls below a certain switching thresholdxe2x80x94generates a switching signal, e.g. binary xe2x80x9czeroxe2x80x9d or xe2x80x9conexe2x80x9d. The switching signal informs the user whether a certain state of the object to be measured is present, for instance, a certain spatial position or a certain pressure.
For many applications, it has to be determined whether the state of the object to be measured lies within certain tolerance limits. Therefore, in order to ascertain these two limit values, two sensors, i.e. a double sensor consisting of a sensor pair, with four connection wires are needed. This calls for quite complex wiring, which is disadvantageous for many applications. In particular in the realm of chemistry, electric lines should be reduced to a minimum. For this purpose, it is known to combine the two control circuits for double sensors on one side, thereby reducing the four connection wires to three. However, when work is done with inherently safe control circuits corresponding to DIN 19234 NAMUR), this has the drawback that the resultant sum current has to be taken into account in the verification of the inherent safety since, with NAMUR which provides that each of two sensors is a two-wire direct current sensor that operates at 8.2 V with switch points operating between 1.2 to 2.1 mA systems, failures such as line breaks or short circuits are checked via the current in the interface between the sensor and the switching amplifier.
Moreover, for numerous applications, more than two states of the object to be measured are to be detected with one system consisting of several sensors. Here as well, two connection wires per sensor or, if applicable, three per sensor pair constitutes quite complicated wiring and there is a need for subsequent electronic devices that are capable of receiving the connection cables and processing the corresponding signals. This is complex in terms of assembly and documentation, and entails potential sources of error.
Another disadvantage of the state of the art is that inductive sensors cannot be mounted at a location too close to a sensor system since they influence each other if they are in operation at the same time, thereby distorting the measured result.
DE 40 33 053 C1 describes a measured value acquisition and transmission device, consisting of a pick-up unit and a detection unit, which can be coupled together inductively for purposes of measured value transmission. The measured value acquisition and transmission device is supposed to prepare and transmit measured data in such a way that, first of all, environmental influences are eliminated to the greatest extent possible and secondly, the measured data can be transmitted interference-free and contact-free, even over long transmission distances. The elimination of environmental influences, for example, high temperatures, is achieved in that the pick-up unitxe2x80x94in addition to a measuring oscillator circuitxe2x80x94also comprises a reference circuit. The oscillation frequency of the measuring oscillator circuit changes primarily as a result of changes in the measured quantity to be monitored, for example, the pressure, but it is also influenced and distorted by environmental influences such as, for example, temperature changes. The reference circuit mimics the measuring oscillator circuit, but it is not capable of reacting to changes in the measured quantity. Its frequency changes only with the environmental influences. Therefore, interfering environmental influences should be eliminated by forming the quotient of the output signal of the measuring oscillator circuit and the output signal of the reference circuit. The quotient is formed in an electronic computing circuit.
The output signal of the electronic computing circuit is transmitted inductively from the pick-up unit to a detection unit. The computing circuit supplies a quasi digital signal that can be transmitted inductively without being very interference-prone. The information on the state of the object to be measured is contained in the frequency of this signal or in the number of counting pulses per unit of time.
The invention is based on the objective of creating a sensor and evaluation system of the type described above, in which the configuration of the individual sensors to form a system is simplified and the wiring effort between the sensors and the switching amplifier is reduced, and which allows an individual monitoring of the operating state of the individual sensor, a close spatial configuration of adjacent sensors without influencing each other and a compact design of the switching amplifier.
With a sensor and evaluation system for acquiring data at a measuring point, especially for double sensors for detecting final positions and limit values, of the type described above, the achievement of the objective is characterized by the following features: a) two sensorsxe2x80x94a sensor pairxe2x80x94are associated together with each of the two connection wires to form the control circuit; b) an alternating current or alternating voltage interface is associated with each sensor pair, and said interface is located in the vicinity of the sensors and is capable of transmitting positive signals to and from one sensor as well as negative signals to and from the other sensor of the sensor pair, whereby the two sensors of the sensor pair are connected together antiparallel and are always operated alternately with respect to each other; c) on the basis of the polarity of the signals, the switching amplifier establishes the association of the sensor signals of a sensor pair that are transmitted via the same connection wires to the sensor in question; d) the current source or voltage source is a current source or voltage source with an alternating polarity.
In another advantageous embodiment of the invention, in order to form the alternating current or alternating voltage interface of the connection wires that establish the connection to the switching amplifier, a short connection wire with a series-connected diode branches off in the vicinity of each of the sensors, whereby each sensor of the sensor pair is connected to two connection wiresxe2x80x94one of them with a diodexe2x80x94having opposite polarity, i.e. the sensors of a sensor pair are connected together antiparallel.
The alternating current/voltage interface is set up with reverse-connect protection diodes that are already integrated into the sensor. Here, the sensors can be electronic sensors or mechanical contacts.
The connection wire leading to the switching amplifier is followed by at least one polarized comparator as an evaluation unit with a storage device and said comparator generates two static signals at the output stage, corresponding to the state of the sensors of the sensor pair.
The levels of the alternating current and the sensors correspond to DIN 19234 (NAMUR) and, for each sensor pair, the current is provided separately to the sensors for an evaluation of the signal for line breaks and line short circuits.
Moreover, the control circuit is galvanically separated from the current source or voltage source and from the evaluation unit and the subsequent elements.
The sensors are polarized analog transmitters and the evaluation units are polarized analog inputs or A/D transducers with storage behavior.
With the sensor and evaluation system according to the invention, in an advantageous manner, the connection wires necessary to connect the sensors to the switching amplifier are reduced from four, optionally three, wires per sensor pair to two per sensor pair. As a result, the wiring effort is reduced and the necessary connection points on the switching amplifier are reduced, as a result of which the latter can be designed more compactly.
With the sensor and evaluation system according to the invention, one sensor pair is actuated by a pair of wires with current having a polarity that alternates over time instead of with direct current. Current having a polarity that alternates over time is also designated as alternating current below, whereby alternating current in the most general sense of the term is to be understood. An alternating current/alternating voltage interface ensures that the individual sensor is supplied with current or voltage having a fixed polarity and that the two sensors of a sensor pair always operate alternately. One of the sensors of a sensor pair is actuated, for example, with the positive or negative half-wave of the alternating current, and the appertaining sensor signal consists of the amplitude of the positive or negative half-wave of the current transmitted to the evaluation unit, i.e. the information on the state of the individual sensor is contained in the amplitude of the particular half-wave of the signal transmitted by the shared control circuit. The evaluation unit associated with each individual sensor is only sensitive to signals of the specific polarity; signals of the opposite polarity are ignored or are evaluated by the evaluation unit that is associated with the other sensor of the sensor pair.
The evaluation unit can be a polarized comparator or a polarized analog input or an A/D transducer. By dispensing with a continuous direct current sensor signal, two pulsed signals can be transmitted via the same pair of lines by feeding in alternating current during the alternating operation of the sensors of a sensor pair. A stationary signal at the output stage is achieved in that the evaluation unit is followed by a storage device, for example, a peak-value hold device, a low-pass filter or a sample-and-hold unit.
In addition to the possibility of a more compact design of the switching amplifier, the cable savings also has the advantage that the entire sensor system can be wired with less effort and is thus easier to maintain and less error-prone.
The wiring according to the invention of two or more sensors to form a sensor system with reduced cabling can also be used when there are two sensors, one of which is used predominantly while the other is only used occasionally. Then, as the alternating current, a signal is used in which the positive and negative half-waves are of different lengths and which is optionally also switched manually.
The alternating current or alternating voltage interface in the area of the sensors can be formed simply in that, in the vicinity of each of the sensors, a short connection wire with a series-connected diode branches off from the connection wires that establish the connection to the switching amplifier, and then each sensor of the sensor pair is connected to two connection wiresxe2x80x94one of them with a diodexe2x80x94having opposite polarity.
In many sensors, especially NAMUR sensors, diodes are already integrated as reverse-connect protection diodes, so that only a fixed input polarity is permissible. In this case, external diodes can be dispensed with when the alternating current interface is set up, and only one branching of the connection wires in the input area of the sensors is necessary in each case, whereby attention has to be paid to the antiparallel switching of the sensors.
The sensor and evaluation system according to the invention can be set up with any kind of sensors, mechanical or electric, analog or digital. The evaluation unit is adapted to the type of sensor (analog/digital operation). The sensor system according to the invention is especially advantageous for installations where inherent safety, e.g. according to DIN 19234 (NAMUR), is required such as, for example, level monitoring of tanks containing explosive liquids or pressure measurement or the detection of a valve state within such an installation. When the operating state of the individual sensor is monitored, for instance, checking for line breaks, the current flow through the appertaining control circuit is measured, whereby this flow must not fall below a certain limit.
In an advantageous manner, with the connection of the sensors according to the invention, the monitoring of the operating state can be carried out sensor-selectively since, with the positive or negative half-wave of the alternating current signal, there are practically two independent signals or pieces of information available.
In order to increase the operating safety of the sensor and evaluation system, it is advantageous for the control circuits to be galvanically separated from the current source or voltage source and from the evaluation units and subsequent elements, for example, with a transformer that, at the same time, transforms the mains voltage down to lower voltages in the range of a few volts, as required by NAMUR.
Since the sensors in the sensor and evaluation system according to the invention function at alternating times as a result of their operation with alternating current, there is less mutual influencing of electronic, e.g. inductive, sensors that are located close to each other than is the case with simultaneous continuous operation with direct current. If there are more than two sensors, the sensors can be connected together about twice as closely to form a sensor system than is the case with operation with direct current since, as a rule, only non-adjacent sensors are operated at the same time and could influence or interfere with each other; however, the individual sensor pairs are not at less distance with respect to each other than in the case of operation with direct current.
In principle, it is also possible for more than two sensors to be operated via a shared control circuit if there are synchronized sampling units before the sensor and in the switching amplifier, so that one sensor is switched on briefly at a given point in time but never at the same time as another sensor, and the analyzed sensor signal is associated with the appropriate output stage. Wiring just two sensors, however, is much less complicated since actuating the sensors and associating the signals can be achieved simply by means of the polarity, and the diodes needed to form the alternating current interface are often already integrated into the sensors.